The present invention relates generally to thermal printing apparatus, and more particularly to an apparatus for correcting a printing data signal to be applied to each of heating elements of a thermal printer, in accordance with a thermal state of each heating element.
In recent years, a number of printers as so-called thermal printer have been developed. With respect to quality of a printed image, most important of these is a printer of the type that printing ink is transferred from an ink sheet to a printing medium by sublimation, in response to heat generated by supplied currents to a print head.
This type of the thermal printer is generally equipped with a print head constructed with heating elements aligned in a main scanning direction. (N.B. A main direction means a transverse direction of the printing medium; meanwhile, a secondary direction means a longitudinal direction of the printing medium.) When a stepwise current is supplied to each of the plurality of heating elements, the temperature of the heating element increases with time delay and this approximates the integration of the input current. That is, the inclination of the temperature of the heating element per unit of time principally depends upon the heat capacity of the heating element and its final value results in being a temperature due to balance between the heat quantity generated by the supply current and the heat quantity running away into the circumference. Accordingly, the integration effect greatly depends upon the structure of the thermal head with the resistance heating elements.
Generally, the time-delay of increase in the temperature of the resistance heating element with respect to the current, i.e., a drive signal, is unavoidable whereby difficulty is encountered to accurately obtain a desirable printing density. As one of techniques for eliminating this printing density problem has been proposed the so-called differentiation correction which cancels the integration effect of the print head by differentiating the input signal to the print head.
For example, in the case of printing an image corresponding to an original image as illustrated in FIG. 1 with a print head having a number of heating elements which are successively arranged in the main scanning direction (traverase direction of the recording medium), a signal produced due to the original image is as illustrated in FIG. 2. In FIGS. 1 and 2, numerals (32 to 256) respectively represent the degrees of densities. If directly applying the signal shown in FIG. 2 to the thermal head, the temperature of the thermal head, i.e., printing density, results the example as shown in FIG. 3 due to the integration effect and does not correspond the original image. Thus, if performing the differentiation correction with a time constant corresponding to the integration effect so as to effecting printing with a waveform as illustrated in FIG. 4, the original image can be reproduced as shown in FIG. 5.
In the aforementioned print head with linearly arranged heating elements, the number of the heating elements reaches, for example, 4096 (400 dots per inch) to cause the size of the print head to become considerably large, and therefore the thermal integration effect, or integration time constant, becomes great. The results of the case of applying a drive signal shorter than the integration time constant will be described hereinbelow with reference to FIGS. 6A to 7B.
If applying a drive signal longer than the integration time constant as illustrated in FIG. 6A, the signal results the waveform as shown in FIG. 6B due to the differentiation correction. Here, at a point a where the printing data signal varies from the high-density state to the low-density state, the temperature of the print head enters into the above-mentioned balanced state. On the other hand, if effecting the differential correction with respect to a signal shorter than the integration time constant as shown in FIG. 7A, the signal results the waveform as shown in FIG. 7B. In this case, at a point a varying from the high-density state to the low-density state, the temperature of the print head does not reach the balanced state and is lower than the temperature corresponding to the point a in FIGS. 6A and 6B. Thus, when effecting the correction similar to the correction made for the FIG. 6A signal, the correction for the low-density portion (edge portion) immediately after the high-density portion becomes excessive so that the density undesirably becomes low. Therefore, it is difficult to reproduce the original image accurately.
The temperature of a given point of the heating element is determined through the balance between the heat quantity generated at the point and the heat quantity lost from the point, and the flow of heat depends upon the heat conductivity of the material at the point and further the temperature difference between the temperature at the point and the temperature at the circumference of the point. Thus, even if supplying the same current to a given heating element, the temperature at the point varies in accordance with the temperature of the circumference of the point, and the temperature at the time of supplying current to a heating element depends on the heat quantity reserved at the point and the circumference, that is, the temperature varies in accordance with the drive signal previously applied.
Although the above description has been made with respect to the secondary scanning direction, the following description is for the main scanning direction. FIG. 8 is an illustration of an original image 3 in which the center portion 1 corresponds to a high density and the circumferential portion 2 corresponds to a low density, and FIG. 9 is an illustration of an printing result obtained by effecting the above-mentioned differential correction with respect to a signal derived from the FIG. 8 original image, In FIG. 9, reference 1' designates a region corresponds to the center portion 1 in FIG. 8 and the portion 1' is reproduced with the same density as the FIG. 8 center portion 1, and references 4d and 4e represent correction-exceeding portions as described with reference to FIG. 7B and the portions 4d and 4e result in becoming lower densities as compared with the portion 2 in FIG. 8. In printing of the region 1', of all the heating elements arranged in the main scanning direction, the heating elements corresponding to the width of the region 1' generate higher heat for the high-density printing as compared with the other heating elements. However, the left and right boundary portions of the higher-heat region tend to cause the heat to be lost due to the temperature difference, and hence the the heat quantity to be lost is more as compared with that of the center portion of the higher-heat region, thereby relatively lowering the temperature of the left and right boundary portions so as to result in the excessive correction over a wide range. Moreover, the temperature of a region 2' contacting with the region 1' becomes high due to the heat from the region 1', thereby printing portions 4b, 4c whose densities are higher than that of the original image. Here, the reason that the portions 4b, 4c are expanded in the secondary scanning direction is that heat is reserved with the passage of time.
Thus, a further improvement is required from the viewpoint of more adequately effecting the correction with respect to the drive signal to the thermal head.